FIG. 41 is a block diagram showing the arrangement of a vector sum phase shifter according to a prior art. FIGS. 42A to 42D are constellation diagrams of signals of the respective portions of the vector sum phase shifter shown in FIG. 41 plotted on a plane.
The conventional vector sum phase shifter includes a 90° phase shifter 1000, two sign inverters 1001I and 1001Q, two variable gain amplifiers 1002I and 1002Q, combiner 1003, and control circuit 1004. This vector sum phase shifter is disclosed in reference, Kwang-Jin Koh, et al., “0.13-μm CMOS Phase Shifters for X-, Ku-, and K-Band Phased Arrays”, IEEE Journal of Solid-State Circuits, vol. 42, no. 11, November 2007, pp. 2535-2546”.
The operation of the vector sum phase shifter shown in FIG. 41 will be described below assuming that an input signal VIN is an ideal sine wave. FIG. 42A shows the input signal VIN. The 90° phase shifter 1000 receives the input signal VIN, and outputs an in-phase signal VINI and a quadrature signal VINQ which are 90° degrees out of phase. On the constellation diagram with an in-phase component (I) plotted along the abscissa and a quadrature component (Q) plotted along the ordinate, the in-phase signal VINI can be represented by only the in-phase component (I), and the quadrature signal VINQ can be represented by only the quadrature component (Q), as shown in FIG. 42B. If the two signals VINI and VINQ are combined, a signal corresponding to a point 220 (at an angle of 45° and an amplitude of 21/2) in FIG. 42B can be obtained.
The in-phase signal VINI and the quadrature signal VINQ are input to the pair of sign inverters 1001I and 1001Q, respectively. The sign inverters 1001I and 1001Q switch, based on the levels of control signals SI and SQ, respectively, between directly outputting the input signal and outputting the signal after inverting the voltage sign. On the constellation diagram, the in-phase signal VINI is output as one of the signal corresponding to the in-phase component (I) and a signal obtained by rotating the in-phase component (I) by 180°, and the quadrature signal VINQ is output as one of the signal corresponding to the quadrature component (Q) and a signal obtained by rotating the quadrature component (Q) by 180°, as shown in FIG. 42C. If the two signals VINI and VINQ are combined, a signal corresponding to one of points 221, 222, 223, and 224 (at an angle of 45°, 135°, 225°, or 315° and an amplitude of 21/2) in FIG. 42C can be obtained.
To obtain finer phase shift angles, the output signals from the pair of sign inverters 1001I and 1001Q are input to the pair of variable gain amplifiers 1002I and 1002Q, respectively. The variable gain amplifiers 1002I and 1002Q change the gains based on the levels of control signals DAI and DAQ, respectively, and consequently change the amplitudes of the input signals and output them. The combiner 1003 vector-combines an in-phase signal VXI and a quadrature signal VXQ output from the pair of variable gain amplifiers 1002I and 1002Q, and outputs the combined signal to the outside as a phase shifter output VOUT.
For example, when the gain on the in-phase signal side is set to 1, and that on the quadrature signal side is set to 0, a signal corresponding to a point 225 (at an angle of 0° and an amplitude of 1) in the constellation diagram of FIG. 42D can be obtained as the phase shifter output VOUT. Similarly, when the gain on the in-phase signal side is set to cos(22.5°)≈0.92, and that on the quadrature signal side is set to) sin(22.5°)≈0.38, a signal corresponding to a point 226 (at an angle of 22.5° and an amplitude of (0.922+0.382)1/2=1) in FIG. 42D can be obtained as the phase shifter output VOUT. When the gain on the in-phase signal side is set to cos(45°)≈0.71, and that on the quadrature signal side is set to sin(45°)≈0.71, a signal corresponding to a point 227 (at an angle of 45° and an amplitude of (0.712+0.712)1/2=1) in FIG. 42D can be obtained as the phase shifter output VOUT.
The above-described three setting examples are operation examples in the first quadrant (0° to 90°). Controlling the pair of sign inverters 1001I and 1001Q allows to obtain a signal having an arbitrary phase and an amplitude of 1 (constant independently of the phase) throughout the four quadrants (0° to 360°). That is, when the gain on the in-phase signal side is set to cos(φ), and the gain on the quadrature signal side is set to sin(φ), a signal having an angle φ and an amplitude of 1 can be obtained as the phase shifter output VOUT.
For the operation of the above-described vector sum phase shifter, the control circuit 1004 receives a digital signal DGTL containing the information of the phase φ to be output, and generates the control signals SI and SQ for the pair of sign inverters 1001I and 1001Q and the control signals DAI and DAQ for the pair of variable gain amplifiers 1002I and 1002Q. The control circuit 1004 includes a digital signal processing circuit (DSP) 1005 which calculates cos and sin (or refers to a memory) to generate the control signals, an encoder 1006 which converts the signal generated by the DSP 1005 into the specific control signals SI, SQ, DAI, and DAQ, and a plurality of digital/analog converters (DACs) 1007I and 1007Q which convert the digital data DAI and DAQ into analog signals to control the variable gain amplifiers 1002I and 1002Q.
Note that the same function as that of the combination of the sign inverters 1001I and 1001Q and the variable gain amplifiers 1002I and 1002Q can be implemented by four-quadrant multipliers (for example, Gilbert cells) (Japanese Patent Laid-Open No. 2004-32446 and Japanese Patent No. 3063093). FIG. 43 shows the arrangement of a vector sum phase shifter in this case. The vector sum phase shifter in FIG. 43 includes a 90° phase shifter 2000, two four-quadrant multipliers 2001I and 2001Q, combiner 2002, and control circuit 2003.
The operation of the 90° phase shifter 2000 is the same as that of the 90° phase shifter 1000. The in-phase signal VINI and the quadrature signal VINQ output from the 90° phase shifter 2000 are represented by the constellation diagram of FIG. 42B.
The four-quadrant multipliers 2001I and 2001Q change the signs and gains of outputs based on the signs and levels of control signals CI and CQ, and consequently change the amplitudes of the in-phase signal VINI and the quadrature signal VINQ and output them, respectively.
The combiner 2002 vector-combines the in-phase signal VXI and the quadrature signal VXQ output from the pair of four-quadrant multipliers 2001I and 2001Q, and outputs the combined signal to the outside as the phase shifter output VOUT. The phase shifter output VOUT is represented by the constellation diagram of FIG. 42D.
The control circuit 2003 receives the digital signal DGTL containing the information of the phase φ to be output, and generates the control signals CI and CQ for the pair of four-quadrant multipliers 2001I and 2001Q. The control circuit 2003 includes a DSP 2004, encoder 2005, and DACs 2006I and 2006Q. In the arrangement shown in FIG. 43, it is necessary to use DACs of differential analog output type as the DACs 2006I and 2006Q in the control circuit 2003.